A colored, positive acting, radiation-sensitive composition for coating lithographic plates is described in U.S. Pat. No. 4,350,753, which issued Sept. 21, 1982. The composition disclosed therein is an admixture of several compounds, including (1) at least one acid-free, aqueous alkali-soluble resin; (2) at least one radiation-sensitive component which upon irradiation yields acidic and/or free radical products; (3) at least one acid and/or radical-sensitive dyestuff which upon contact with the above radiation-sensitive component is rendered colorless less intensely colored, or changed in color; and (4) at least one positive-acting component.
Examples of the above components and the processes for making the same are extensively described in U.S. Pat. No. 4,350,753; columns 3, 4 and 5 of which are incorporated herein by reference.
In this patent, there is a disclosure of the preferred use of "novolak resins", which for the purpose of definition shall be herein defined as the low molecular weight condensation products of mono- or polyhydroxy aryl compounds with aldehydes and ketones. A commercial example of a suitable novolak resin is Alnoval PN 430, manufactured by Hoechst.
In the commercial preparation of novolak resins, and particularly the Alnoval PN 430 the product contains free acid, which when used in lithographic coating compositions decreased the contrast between the exposed and unexposed areas of the lithographic plate. It was desirable, if not necessary, to neutralize these resins such that they became substantially acid-free before introduction into the radiation-sensitive or photosensitive composition. This has been accomplished previously by initially treating a propanol solution of the novolak resin with a base and then precipitating the base-treated novolak resin from a cold dilute solution of sodium chloride. The precipitated novolak resin is then dried and stored for future use.
One of the disadvantages associated with the foregoing neutralization process was the enormous increase in the cost of producing the novolak, which was raised at least five-fold by the separate neutralization treatment and recovery steps. In addition to the increase in production costs there is also the problem of disposing of the waste products obtained from the neutralization treatment.
In accordance with the present invention, it how has been discovered that the novolak resin can be effectively neutralized in situ, i.e., the resin can be alkali treated in solution during the formulation of the lithographic plate coating composition without encountering untoward effects.
The novolak resin is dissolved in a solvent that is common to the other coating composition components: the positive sensitizer, the color changing dye, and the light activated acid-releasing compound. A common solvent for this purpose is n-amyl acetate, and for most purposes the novolak resin is dissolved in the n-amyl acetate solution by slowly adding the novolak resin to the n-amyl acetate solution while stirring. The novolak resin is then neutralized by adding a solution containing both triethylamine and n-amyl acetate. The mix is stirred for 30 minutes to complete the neutralization. The alkali-treated novolak resin is then capable of being directly mixed with the other components of the lithographic composition.